The present invention relates to a switching method of transmission lines in a transmission network. Specifically, the present invention relates to a method of transmission line switching, transmission equipment, and network architecture suitable to SONET (Synchronous Optical Network) or SDH (Synchronous Digital Hierarchy) network.
Recent years, there are proposed many transmission line switching methods to protect signals against line failure (for example, inadvent disconnection or degradation of line failure of repeaters) in order to improve the reliability of transmission services.
These methods comprise for example: (1) 1 to N type NPS (Nested Protection Switching) network in which a plurality of working lines and protection lines are installed in a same path, and line switching method thereof, (2) 4-Fiber BLSR (Bidirectional Line Switching Ring) and transmission line switching method thereof, in which a plurality of transmission equipment are connected by the working line and protection line in a ring form. Examples of the former method include xe2x80x9cxe2x80x9cNested Protection Switchingxe2x80x9d T1X1.5/90-132,1992xe2x80x9d and Fiber Network Service Survivability, and the examples of the latter include xe2x80x9cBellcore xe2x80x9cSONET BLSR Genetic Criteriaxe2x80x9d GR-1230-CORE,1993xe2x80x9d.
FIG. 9 shows an example of N-type NPS network. In this figure, 101 through 104 designate transmission equipment. The network example of FIG. 9 is connected as follows: its working line 105 is terminated by the transmission equipment 101 and 102. A working line 106 is terminated, on the other hand, by the transmission equipment 102 and 103. These working lines 105 and 106 are connected by using an Add-Drop Multiplexing equipment in the transmission equipment 102. A working line 107 is terminated by the transmission equipment 102 and 104. And the working line 107 is routed by the transmission equipment 103.
On the other hand; the protection lines 109 through 111 are depicted in FIG. 9 by dotted lines. The protection lines 109 through 111 are all connected to every transmission equipment 101 through 104, being connected by using Add-Drop Multiplexing equipment 114 within respective transmission equipment. Each of transmission equipment has ability of switching between transmission lines, and therein the working lines and protection lines transmit signals bidirectionally.
One to N type NPS network as shown in FIG. 9 may select Add-Drop Multiplexing equipment or repeater for a transmission equipment when required for a working line. This allows the flexibility of the N-type network service to be improved. Also its economical efficiency may be improved, since N working lines share the protection line in this network. Furthermore, this network is predominant in the addition of working lines. For example, when traffics between the transmission equipment 101 and 103 are required to be newly added, it is possible to add working lines of the required capacity easily. Thus, as in the example of working line 108, the line addition may be realized by terminating by the transmission equipment 101 and 103, and by repeating by the transmission equipment 102.
At this point, how to switch when a failure occurs in such an architecture will be described with reference to FIG. 9. The switching method is dependent on following three factors: (1) the position in the transmission equipment at the point where the failure has been occurred; (2) the level of importance of the failure; and (3) the order of the occurrence of failures.
If the first failure of the importance level 3 has been occurred in the working line 105, the working line will be protected by using the protection line 109. In this case the larger the importance level, the faster the protection of the failure will be realized.
If the second failure of the importance level 1 has been occurred in the working line 106, the working line will be protected by using the protection line 110.
If the third failure of the importance level 2 has been occurred in the working line 108, the protection lines 109 and 110 will be required for the protection. However, in this case, the protection lines 109 and 110 are already in use. By comparing the importance level between failures in the protection lines, the importance level of the protection line 109 is three and that of the protection line 110 is one. As the importance level of the protection line 109 is higher than the importance level of failure of the working line 108, the working line 108 will not be protected. In this case the working line 106 will remain protected. Thus the transmission equipment which has detected the failure of the working line 108 should know the working line 108 is denied being protected.
If the fourth failure of the importance level 4 has been occurred in the working line 107, the protection lines 110 and 111 are required for the protection while the protection line 110 are already in use. When referring to the importance level of that protection line, the importance level of the protection line 110 is one, which importance level is lower than the importance level of the working line 107. Thus the protection line 110 will be used for the protection of the working line 107. At this time the fourth failure will be protected, whereas the second and third failures will not.
As described above, the switching decision and switching operation between transmission lines in an NPS network will be done in the transmission equipment which terminates the working line. This means that the transmission equipment should know the information on other transmission lines that the working line requests as a protection line simultaneously. Therefore, whether or not the switching operation is proper should be determined correctly based on the communication of switch control information among respective transmission equipment.
There are proposed such methods as follows, in which the switching operation is to be performed by exchanging the control information in the transmission equipment based on the overhead of SOMET/SDH. These include: (1) a method using Automatic Protection Switching bytes (APS bytes) and DCC bytes (e.g., ITU-T(International Telecommunication Union-Telecommunication Standardization Sector), T1X1.5/90-132,1990); and (2) a method using APS bytes and a timer (Tsong-Ho Wu,xe2x80x9cFiber Network Service Survivabilityxe2x80x9d,Aretec house,1992 ). In this context the APS bytes indicates the bytes defined in the SONET/SDH for the use of exchanging of control information for transmission line switching on the SOMET/SDH. APS bytes are comprised of so-called K1 byte and K2 byte. The use of APS bytes on a Point-to-Point basis may be found in the section 5 of xe2x80x9cBellcore GR-253-CORE,xe2x80x9d issue Dec. 1, 1993.
Now, SONET, SDH and a network of the present invention conduct digital transmission by using an overhead of transmission frames for digital transmission and by using performing frame phase alignment and stuff control by swapping pointers in the digital transmission, as known well.
The above described first switching method xe2x80x9cT1X1.5/90-132xe2x80x9d is a method for an appropriate switching of working lines on the basis of comparison of the importance level by transmitting the importance level of the working line using a plurality of DCC bytes.
The above described second switching method xe2x80x9cFiber Network Service Survivabilityxe2x80x9d is a method as follows. The transmission equipment having detected a failure transmits K1 bytes of APS bytes to wait for the response with K2 bytes. The destination node transmits K2 bytes indicating the response when K1 bytes are received, on the assumption that a protection line has been allocated. The source node receiving the K2 bytes indicating this response starts the switching operation. If there exists a request of higher importance level on the route to the destination node, the K1 bytes will not be arrived at the destination node, so that the K2 bytes indicating the response will not be transmitted. Therefore, a timeout will occurr in the transmission equipment which has detected a failure, so that the switching operation will be determined not to be performed.
FIG. 10 illustrates 4-fiber BLSR, one of transmission methods. In FIG. 10, reference numerals 115 through 118 designate transmission equipment. The working lines 119 through 122 and protection lines 122 through 125 are connected in a ring form. Each of these transmission equipment has ability of switching transmission lines such,that these transmission equipment transmit bidirectionally signals on the working lines and the protection lines.
Now the basic operation of transmission line switching against failures of lines in this 4 fiber BLSR will be described below. In FIG. 10, when a failure occurs in a working line 122, signals will be protected by using protection line 126. Also, if there are failures on both of the working line 122 and the protection line 126, a detour route is used which is specific to the ring form. That means that in the transmission method of 4-Fiber BLSR signal protection may be achieved by using the protection lines 123 through 125. The protection method in the ring form is characterized in that two routes of clockwise and counter-clockwise directions may be selected. For the transmission method of the 4-Fiber BLSR, there has been proposed a high-speed switching method using solely the APS bytes (Bellcore xe2x80x9cGR-1230-CORExe2x80x9d, Issue Dec. 1, 1993) in the prior art.
Simultaneous Digital Hierarchy (SDH) is standardized in order to achieve the improvement of network, the flexible multiplexing of various information, and the provision of multi-vendor telecommunication devices. In SDH, data will be transmitted on the frame basis, and for the overhead of transmission frames APS bytes (K1 and K2 bytes) are allocated to the control signals of line switching as described above.
APS bytes on the PTP (Point-to-Point) scheme and Ring scheme are standardized as shown in FIG. 31. This allows line switching of both linear and ring networks.
However, since the NPS network is a multi-line multi-node network, the previous APS bytes cannot be applied thereto. In addition, since a plurality of working lines are allocated to one protection line, an effective protection against multiple failures should be considered.
The problems of the switching method of the NPS network will be further described in greater details below.
First, when using APS bytes and DCC, there are problems that the switching is very complex and takes times for determining since a plurality of DCC are processed by one single controller.
Second, there are problems that when using APS bytes and a timer, the method is low in speed and its reliability is poor, since the signals from the source node indicating response are either arrived or not. Furthermore, all of the working lines and protection lines should pass through an identical path, resulting in the limitation of the installation of the transmission equipment and the transmission lines.
Also the BLSR as proposed above has following problems: In the BLSR which is a one to one system, protection lines corresponding to the transmission capacity of the working lines have to be installed. Therefore, the capacity of the whole ring should be determined by the capacity required for a span of maximum traffic. In FIG. 10, when only the traffic between the transmission equipment 115 and 116 is the maximum capacity in the network, the working line 119 is required to set the transmission capacity of the defined optic fibers to the maximum value as well as to build the transmission lines of whole ring at its maximum capacity value. As such, the capacity of whole network should be increased in order to augment only the traffic in a specific segment, so that there are problems on the effectiveness and the economic performance of the transmission lines.
As have been described above in greater details, switching of NPS network requires information about the number and the priority of working lines, and about the source and the destination nodes of the switching signals. If the switching is achieved by using APS bytes as used in the PTP or Ring schemes, the APS bytes capacity will be insufficient.
The main object of the present invention is to provide a network capable of realizing high speed transmission line switching method, transmission equipment to be used in said network, and a method for control thereof.
Now the overview of the present invention will be briefly summarized, and then various aspects of the present invention disclosed herein will be described hereinbelow in greater details.
The basic concept of the present invention is to allocate the number of the working lines, its priority, and the source node number of the switching signals among these items of information to the APS bytes. In addition a network table for storing the connecting status of networks and the failure information will be provided for each of nodes, thereby the destination node may be identified based on the working line number, the source node number which are included in the received APS bytes, and the connecting status of the network table.
An example of allocation of said various information in APS bytes is shown in FIG. 32. There are respectively two types of information items which are allocated to respective K1 bytes and K2 bytes in the APS bytes. For example, (1) priority, and (2) working line number are allocated to the K1 byte, and (1) source node number, and (2) switching status are allocated to the K2 byte. Of cource, (1) priority, and (2) working line number may be also allocated to the K2 byte, and (1) source node number, and (2) switching status may be also allocated to the K1 byte.
Also, in the present method APS bytes may be used not only for transmission line switching but also may be used for information update of the network table in other nodes so as to achieve rapid switching determination.
Here, the basic switching protocol of the present invention will be described below.
FIG. 33 shows the most basic switching protocol, in which, if a failure occurs at the time T1 in the working line # 1, the node B detects the failure to send a bridge request to the node A. The node A verifies whether the protection line # 0 has been insured to be used, then performs switching of transmitter side and informs it to the node B. Next, the node B switches the transmitter side and the receiver side then informs the node A of the completion of switching. The node A also switches the receiver side. All switching operation will be completed at the time T2.
Then, the switching protocol of the switching of multiple failures in accordance with the present invention. FIG. 34 shows an example of multiple failures. Here it is assumed that the line # 1 (failure SD) and line # 3 (failure SF) are already protected by using protection lines, and another failure SF has been further occurred in the line # 2. Here, following two rules should be defined: The SF (signal failure) should be higher priority than the SD (signal deterioration) [rule 1]. When the priority is the same, the switching of the line already protected will be retained [rule 2].
In the conventional switching method, the protection line between the nodes A and B, which protected the line # 1 will stop protecting the line # 1 according to the rule 1. However the protection line between the nodes B and C will continue to protect the line # 3 according to the rule 2. Therefore, both the line #1 and #2 will not be protected, so that the protection line between the node A and B will not be used effectively.
In accordance with the method of the present invention, the effective rate of protection lines will be improved by maintaining current protection and by exchanging information of failures newly occurred between nodes.
To achieve the method in the present invention, the number of the working line in which a failure has been newly occurred will be allocated. to the K1 byte for that APS bytes. And the priority of the currently protected working line and that of the working line of the newly occurred failure are multiplexed on the K1 bytes (for example, the SF on SD in FIG. 32).
Also, non-protectable SF should have lower priority than the SD [rule 3].
By exchanging failure information between nodes A and B by using the multiplexing function of the present invention, the protection line between nodes A and B which protected the working line #1 will continue to protect the working line #1 according to the rule 3. As a result, the protection of the line #2 may not be performed whereas the protection of the line #1 may be maintained, so that the protection line between nodes A and B may be efficiently used.
The conventional switching method has no compatibility between the PTP and Ring form. However, in accordance with the method of the present invention, if the form of the network changes from the linear one to the ring one, due to for example the addition of new nodes, it will be sufficient to solely update the Network Table. In this manner, the self-protecting method according to the present invention may use the network management software.
In addition, the self-protecting method according to the present invention may use the protection lines so as to achieve efficient protection since the protection lines are switched over after it is insured that the protection of failed line in case of multiple failures.
In addition, the self-protecting method according to the present invention may realize effective use of protection lines, as the switching to the protection lines is performed after the protection of failed lines is verified to be insured in case of multiple failures.
In addition, the self-protecting method according to the present invention may realize more reliable switching since it inherits the PTP method characterized by the one to N switching as well as the Ring method characterized by the Ring switching. In this manner, the self-protecting method accordance with the present invention may of course apply to the so-called NPS(Nested Protection Switching) type network.
Next, an exemplary embodiment of the present invention disclosed herein will be described in overview.
The network of the present example is a network comprising K transmission equipment (where K is integer not less than three), (Kxe2x88x921) protection lines connecting said transmission equipment in a straight chain form, and a plurality of working lines connecting said transmission equipment, and having overhead in transmission frames, characterized in that when a failure occurs in one of the working lines, all of the switch control information concerning the failure may be exchanged among the transmission equipments, more preferable all of the transmission equipments, the transmission equipment of the network by using automatic switching bytes in the overhead of the transmission frames, for switching the transmission lines based on the switch control information.
The overhead comprised the transmission lines means the area for transmitting operational maintenance information of the network. Also, the automatic switching bytes in the overhead is used for exchange of signals for controlling the system switching among transmission terminal devices, as well as for display of alert status. In a SONET or SDH network, the automatic switching bytes is called as APS bytes, which in general is comprised of two areas of K1 byte and K2 byte.
Thus the present invention may be very useful to be applied to the typical SONET or SDH network
The typical network according to the present invention is a SONET or SDH network comprised of K transmission equipment (where K is integer not less than three), (Kxe2x88x921) protection lines connecting said transmission equipment in a straight chain form, and a plurality of working lines connecting said transmission equipment, characterized in that when a failure occurs in one of the working lines, all of the switch control information concerning the failure may be exchanged among the transmission equipments of the network, more preferable all of the transmission equipment of the network, by using APS bytes in the overhead of the SONET or SDH frames, for switching the transmission lines based on the switch control information.
Another aspect of the present invention is a Ring form as follows: it includes a network K transmission equipment (where K is integer not less than three), K protection lines connecting said transmission equipment in a ring form, and a plurality of working lines connecting said transmission equipment, and having overhead in the transmission frames, as well as at least one of the transmission equipment is connected to three or more working lines, characterized in that when a failure occurs in one of the working lines and the protection lines, all of the switch control information concerning the failure may be exchanged among all of the transmission equipment of the network by using automatic switching bytes in the overhead of the transmission frames, for switching the transmission lines based on the switch control information.
Therefore, the present invention may be very useful to be applied to the typical SONET or SDH network in case of Ring form as well.
Next, the transmission equipment according to the present invention have following structure:
Transmission equipment comprising:a connecting unit for a plurality of working lines, a connecting unit for a plurality of protection lines, a monitoring unit for monitoring anomalies in said working lines, a transmitter/receiver for information stored in the area for automatic switching in the overhead for exchanging failure information with adjacent transmission equipment through a protection line, a Network Table for storing the connection status of the network and failure information, a processing unit for setting the transmission line switching and for notifying the setting of the transmission line switching to said adjacent transmission equipment, based on the information stored in said Network Table, the information of the results of monitoring transmission lines by said monitor, and the information stored in the area for automatic switching in the overhead.
In addition, the data indicating said network structure may comprise connection information among the transmission equipment of the working lines, connection information among the transmission equipment of the protection lines, transmission equipment number assigned to a working line, and current failure information of the transmission line in the network.
In Ring form network, the data indicating said network structure may comprise connection information among the transmission equipment of the working lines, connection information among the transmission equipment of the protection lines, transmission equipment number, path information of the protection line used for protecting signals of the working lines, number assigned to a working line, and current failure information of the transmission line in the network.
The transmission equipment according to the present invention may be very useful to be applied to the typical SONET or SDH network. And it may be also applied to the liner form, the Ring form, and NPS form of the network.
In the transmission equipment according to the present invention said data indicating the network structure may be dynamically updated by the signal from the processor.
Next, various embodiments of the transmission line switching method according to the present invention will be described.
One embodiment of the transmission line switching method according to the present invention is a method for controlling transmission line switching which may be performed by the processor of the transmission equipment used in a network which comprises K transmission equipment (where K is integer not more than three), Kxe2x88x921 protection lines connecting said transmission equipment in a straight chain form, and a plurality of working lines connecting said transmission equipment, comprising the steps of:
(1) first, analyzing the contents of received APS bytes;
(2) second, processing transmitting APS bytes according to the results of the analysis;
(3) third, updating a Network Table according to the results of the analysis; and
(4) fourth, setting the direction of transmission of the processed APS bytes.
Another embodiment of the transmission line switching method according to the present invention is a method for controlling transmission line switching which may be performed by the processor of the transmission equipment used in a network which comprises K transmission equipment (where K is integer not more than three), K protection lines connecting said transmission equipment in a ring form, and a plurality of working lines connecting said transmission equipment, at least one of the transmission equipment being connected with three or more working lines, comprising the steps of:
(1) first, analyzing the contents of received APS bytes;
(2) second, processing transmitting APS bytes according to the results of the analysis;
(3) third, updating a Network Table according to the results of the analysis; and
(4) fourth, setting the direction of transmission of the processed APS bytes.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows, and will be described based on the above described two basic methods of controlling transmission line switching.
Thus, in the above two methods of controlling transmission line switching, said second step includes the step of generating request signal for triggering switching, and said fourth step includes the step of setting the transmission direction of the request signal in the path used for the protection, based on the data stored in the Network Table.
In addition, still another embodiment of the method for controlling transmission line switching according to the present invention is as follows: in the above two methods of controlling transmission line switching, said second step includes the step of generating status signal indicating switching status, and said fourth step includes the step of setting the transmission direction of said status signal in the path not used for protection.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows: in the above two methods of controlling transmission line switching, if the APS bytes received in the first step is a request signal indicating switching trigger, and bridging request, and is sent to that equipment, said second step includes the step of generating APS bytes indicating response to the bridge request of the transmission equipment.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows: in the above two methods of controlling transmission line switching, if the APS bytes received in the first step is a request signal indicating switching trigger and bridging request, and is sent to that equipment, said second step includes the step of generating APS bytes indicating protection request of the transmission equipment.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows: in the above two methods of controlling transmission line switching, if the APS bytes received in the first step is a request signal indicating switching trigger and protection request, and is sent to that equipment, said second step includes the step of generating APS bytes indicating protection request of the transmission equipment.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows: in the above two methods of controlling transmission line switching, if the APS bytes received in the first step is sent to another equipment and is a request signal indicating switching trigger, when that transmission equipment performs switching, said second step includes the step of regenerating APS bytes.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows:
Thus, in the above two methods of controlling transmission line switching, when detecting or receiving a plurality of failure information at a transmission equipment, said second step includes the step of generating APS bytes multiplexing said plurality of failure information.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows:
Thus, in the above two methods of controlling transmission line switching, if the APS bytes received in the first step is sent to another equipment, and is request signal indicating switching trigger, and if the protection line opposed to the direction of receiving is already used for the protection of another failure, and if the importance level of the received APS bytes is higher than that of the protected failure, said second step includes the step of generating APS bytes multiplexing information on the fact that the switching of working line protected from the failure has been completed, and bridging request of the received APS bytes, and said fourth step includes the step of setting the direction of transmission of the generated APS bytes as to the direction opposed to the receiving direction of the APS bytes. Furthermore, if the APS bytes received in the first step is sent to another equipment, and is request signal indicating switching trigger, and if the protection line opposed to the direction of receiving is already used for the protection of another failure, and if the importance level of the received APS bytes is lower than or equal to that of the protected failure, the second step includes the step of generating APS bytes including switching reject, and the fourth step includes the step of setting the direction of transmission of the generated APS bytes as to the receiving direction of the APS bytes.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows, and will be described based on the above described two methods of controlling transmission line switching.
Thus, in the above two methods of controlling transmission line switching, if the APS bytes received in the first step is sent to another equipment and is request signal indicating switching trigger, and if the protection line opposed to the direction of receiving is already used for the protection of another failure, and if the importance level of the received APS bytes is higher than that of the protected failure, said second step includes the step of generating APS bytes multiplexing information on the fact that the switching of working line protected from the failure has been completed, and switching reject of the received APS bytes, and said fourth step includes the step of setting the direction of transmission of the generated APS bytes as to the direction opposed to the receiving direction of the APS bytes.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows: in the above two methods of controlling transmission line switching, if the APS byte received in the first step indicates switching reject, and is determined to be sent to that equipment, said third step includes the step of decreasing the importance level of request causing said switching reject, and updating data in the Network Table.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows: in the above two methods of controlling transmission line switching, if the APS byte received in the first step indicates switching reject, and is determined to be sent to that equipment, said third step includes the step of determining whether or not there is a different path from the path of the protection line used for protection stored in the Network Table, and if there is another path, updating the path information of the protection line used for signal protection of the working line stored in the Network Table.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows: in the above two methods of controlling transmission line switching, if the APS bytes received in the first step indicates switching reject, and is determined to be request signal indicating switching trigger, when that transmission equipment does not perform switching, the second step includes the step of transferring the received APS bytes as transmission APS bytes without changing.
Still another embodiment of the method for controlling transmission line switching according to the present invention is as follows: in the above two methods of controlling transmission line switching, if the APS bytes received in the first step is determined to be status signal indicating switching status, the second step includes the step of transferring the received APS bytes as transmission APS bytes without changing.
Furthermore, the method for controlling transmission line switching according to the present invention includes APS bytes including the number of the working line of the highest importance level, the number of source node of the signal, and the response status of switching in the source node.